1. Field of the Invention
The invention relates to low molecular weight polyvinyl acetals with improved adhesion, a process for their preparation, and also their use.
2. Background Art
The preparation of polyvinyl acetals, which are obtained from the corresponding polyvinyl alcohols by polymer-analogous reaction with the appropriate aldehydes, have disclosed as early as 1924, and since then a wide variety of aldehydes has been used for preparing the corresponding polyvinyl acetals. Polyvinyl acetals are prepared in a 3-stage process wherein a polyvinyl acetate polymer is first prepared, hydrolyzed to a polyvinyl alcohol polymer, and subsequently acetalized. The resultant products contain vinyl alcohol units and vinyl acetate units as well as vinyl acetal groups. Polyvinyl formal, polyvinyl acetacetal, and polyvinyl butyral (PVB) have achieved particular commercial importance. The term “modified polyvinyl acetals” hereinafter refers to polyvinyl acetals which contain other monomer units as well as vinyl acetate, vinyl alcohol, and vinyl acetal units.
The largest application sector for polyvinyl acetals is the production of safety glass in automotive construction and for architectural uses, where plasticized polyvinyl butyral films are used as an intermediate layer in glazing units. Mixtures containing modified polyvinyl butyral have also been proposed for this purpose, for example those having the acetal units described in EP-A 368832 with sulfonate, carboxylate, or phosphate functionality, which feature improved blocking performance and flow performance. EP-A 634447 discloses modified polyvinyl butyrals whose main polymer chain contains monomer units which bear sulfonate groups, these polyvinyl butyrals being obtainable via acetalization of polyvinyl alcohols which have sulfonate functionality.
EP-A 461399 discloses polyvinyl butyrals modified with amino groups, used as precipitants. Another application sector for polyvinyl butyrals is their use in anti-corrosion coatings, as disclosed by EP-A 1055686, for example, where use is made of polyvinyl acetals modified with tertiary alkanolamines.
Polyvinyl butyrals are also used as binders in paints, and specifically as binders in printing inks, their success in these applications being due to their good pigment binding power. A requirement of printing ink applications is a very low solution viscosity of the organic solutions of the polyvinyl butyrals, in order to permit formulation of inks with high solids content, using a very high binder content. Examples include the modified polyvinyl butyrals disclosed in DE-A 19641064, obtained by acetalizing a copolymer having vinyl alcohol units and 1-alkylvinyl alcohol units.
A disadvantage possessed by all of the polyvinyl acetals described in the prior art is their inadequate adhesion to specific substrates. For this reason, addition of coupling agents is essential in many cases. EP-B 0346768 describes the coating of films or foils with amino-functional silanes, such as N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, in order to improve bonding to other polymers, in particular to polyvinyl butyrals. In the case of polyethylene films and polyester films, these are coated either with vinyltrimethoxysilanes or with chloropropyltrimethoxysilanes in order to improve adhesion, providing successful lamination using hot-melt adhesives based on ethylene-vinyl acetate copolymer (E. Plueddemann, “Bonding through Coupling Agents”, Plenum Press, New York, 1985). Use is also made of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane as a primer coat for improving the adhesion of an ionic resin, i.e. a salt of a polymer based on ethylene-methacrylic acid, to glass or polycarbonate films (U.S. Pat. No. 4,663,228). EP-B 0636471 claims a process for producing a glass composite, where the adhesion between glass and polymer film, i.e. polyvinyl butyral, is improved using a mixture of two or more silanes.
The processes described in the prior art for improving adhesion between polyvinyl acetals and critical substrates by adding known coupling agents have clear disadvantages. For example, the adhesion-promoting action is often not sufficiently long lasting, or the composite weakens over time. Another disadvantage is the yellowing of aminosilanes and their unpleasant odor and skin-irritant action, a known problem when using amino-functional compounds. Another problem is the addition of the coupling agent per se. This process always requires an additional, time-consuming step in the mixing procedure. The selection and addition of suitable coupling agents, furthermore, requires great expertise, since when the adhesion promoter is added to a polyvinyl acetal, for example in an organic solvent, the result can be incompatibility and inhomogeneity, leading to phase separation. In addition, the reactive silanes subsequently added as coupling agent may become involved in side reactions, for example hydrolysis or condensation reactions. Another disadvantage consists in the large amount of adhesion promoter which ordinarily must be added in order to achieve noticeably better adhesion of polyvinyl acetals on critical substrates. As a result, the process can become very expensive, since the price of silanes is high.